1. Field of the Invention
The present invention relates to a thread feeding apparatus for a sewing machine, and particularly to a thread feeding apparatus for sewing machine in which a thread can be supplied to a stitch forming device for forming a stitch in a cloth without necessity of an operation of a stitch balancing thread tension.
2. Description of the Prior Art
The conventional lock stitch sewing machine or overlock sewing machine is provided with a thread tension device comprising a pair of thread tension discs held between thread tension springs at a predetermined pressing force, thereby holding a thread between the thread tension discs and adjusting a tensional force generated in the thread, at the time of forming a stitch by a needle or a looper, to a predetermined value by means of the thread tension device so that a stitch can be neatly finished even if the type and thickness of the thread is changed. In this case, feeding of the thread is performed by drawing by the thread tension device together with a needle thread take up or a looper thread take up and tightening stitches. In these sewing machines, a significantly complicated manual operation input for adjusting the thread tension device is necessary. Particularly, in the overlock sewing machine, since the number of threads is increased, it is significantly complicated to obtain a good balance of the tension between the threads.
Further, there is known a computer controlled overlock sewing machine which is provided with a pair of rollers each having an encoder and an electro-magnetic thread gripper so that the encoder measures a length of the thread, thereby opening and closing each of the electro-magnetic thread grippers so as to supply a thread quantity necessary for forming a stitch (Japanese Patent Unexamined Publication No. 2-45088). The computer controlled overlock sewing machine has a complex mechanism and requires a manual operation for prior input of data concerning the type of the thread, thickness of the thread, cloth thickness, stitch width and the like, so as to determine the quantity of the thread, creating the problem that it is difficult to correlate the manual input to the finish of the stitch. Still further, in the computer controlled overlock sewing machine, since the roller forcibly supplies the thread even when the thread is cut, thread waste is fed.
In order to solve the above problem, the applicant of this application has suggested a thread feeding apparatus for a sewing machine which can provide a suitable stitch balancing thread tension (Japanese Patent Unexamined Publication No. 9-19583). Since the thread feeding apparatus for a sewing machine employs a thread storing device which stores a thread quantity necessary for a stitch and removes the thread at the time of stitch forming, there are the problems that the mechanism becomes complex so that it is difficult to provide accuracy in manufacturing, durability thereof is low and the apparatus can not provide high speed sewing. Further, the thread feeding apparatus for a sewing machine can not satisfy consumer demand for a number of needles and a narrow hem or rolled hem sewing.